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Cellulose Sponge Supported Palladium Nanoparticles as Recyclable Cross-coupling Catalysts Yingzhan Li, Lei Xu, Bo Xu, Zhiping Mao, Hong Xu, Yi Zhong, Linping Zhang, Bijia Wang, and Xiaofeng Sui ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 9, 2017

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Cellulose Sponge Supported Palladium Nanoparticles as Recyclable Cross-coupling Catalysts Yingzhan Li a, Lei Xu b, Bo Xu b, Zhiping Mao a*, Hong Xu a, Yi Zhong a, Linping Zhang a, Bijia Wang a,Xiaofeng Sui a* a

Key Lab of Science and Technology of Eco-textile, Ministry of Education, Donghua University, Shanghai 201620, People’s Republic of China. b College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai 201620, People’s Republic of China.

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ABSTRACT Robust and flexible cellulose sponges were prepared by dual-crosslinking cellulose nanofiber

(CNF)

with

γ-glycidoxypropyltrimethoxysilane

(GPTMS)

and

polydopamine (PDA) and used as carriers of metal nanoparticles (NPs), such as palladium (Pd). In situ growth of Pd NPs on the surface of CNF was achieved in the presence of polydopamine (PDA). The modified sponges were characterized with FT-IR, XRD, EDX, SEM, TEM and TGA. XRD, EDX and TEM results revealed that the Pd NPs were homogenously dispersed on the surface of CNF with a narrow size distribution. The catalysts could be successfully applied to heterogeneous Suzuki and Heck cross-coupling reactions. Leaching of Pd was negligible and the catalysts could be conveniently separated from the products and reused.

KEYWORDS Cellulose nanofiber, Sponge, Polydopamine, Palladium, catalysts

1. INTRODUCTION Precious metal catalysis has been and continues to be the predominant way of constructing C-C, C-H, and C-heteroatom bonds in organic synthesis.1 Palladium (Pd) has been applied as an indispensable catalyst of many organic transformations2 among which palladium-catalyzed cross-coupling reactions have attracted broad attentions due to their strong impact on materials and pharmaceuticals.3-7 Due to the tedious separation and recovery of the rather costly Pd complexes used in homogeneous catalysis,8-11 much efforts have been devoted to developing organic and inorganic supports to immobilize Pd catalysts.12-16 For example, Kadib and co-workers used porous chitosan nanofibrillar microspheres as palladium carrier and used it to efficiently promote Sonogashira cross-coupling reactions.2 Uyama et al11 reported the use of Pd NPs captured in mesopourous polymeric monoliths in Suzuki cross-coupling reactions. Huang and co-workers17 employed carboxyl-containing microporous nanotube networks to anchor Pd NPs. Moores12 and Tam18 also prepared Pd NPs supported by nanocellulose. 2

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Nanocelluloses have drawn broad attentions for being extremely abundant and versatile, with potential applications spanning a variety of industries.18-21 these materials are composed of β(1−4) linked D-glucose units, which allow for conveniently tunable surface chemistry.22 They can also be structurally engineered into forms spanning from membranes, fibres, microspheres to sponges.23-29 The use of nanocellulose-based porous materials as metal catalyst supports is appealing for a number of reasons: (1) facile preparation and modification of the CNF sponges; (2) efficient heat and mass transfer enabled by the open-cell structure and large surface area; (3) convenient separation of the supported catalysts from reaction mixtures enabled by the monolithic structure; and (4) their superior environmental friendliness, availability, and decent stability both in water and most organic solvents. However, the unmodified cellulose sponge has poor mechanical properties. In this work, the mechanical properties of porous cellulose materials were improved by dual-crosslinking

with

γ-glycidoxypropyltrimethoxysilane

(GPTMS)

and

polydopamine (PDA). GPTMS contains reactive trimethoxysilane groups that enable it to form covalent bond to the hydroxyl groups of cellulose and form composite silsesquioxane

on

the

fibre

surface

through

hydrolysis/condensation.30

Its

epoxypropoxy groups also react with the amino groups on PDA, forming stable chemical cross-links.31 Recently, it was shown that hydrolysis/condensation of silane agents and self-polymerizaiton of dopamine might be achieved simultaneously in an alkaline aqueous solution.

32-33

PDA could be incorporated in the networks of

silsesquioxane via hydrogen bounding and physical entanglement leading to enhanced mechanical properties of the cellulose sponge. The thus modified cellulose sponge is a fascinating polymer featuring high density of functional groups such as indolylamines, quinones, catechols and imines.

32-33

Furthermore, the catechol moiety

in PDA could reduce metal ions without adding any external reducing agent,34-36 so that metal nanoparticles could be firmly anchored adsorbed on the surface. Pd has been successful immobilized on Fe3O4,32 Carbonized loofah and silica37 gel with the assistance of PDA. 3

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We describe an in situ strategy to firmly immobilize Pd NPs on cellulose sponges. The functionalized sponge was successfully used in catalyzing Suzuki and Heck cross-coupling reactions. The present strategy allows a convenient access to recyclable leaching-proof heterogeneous metal catalysts.

2. EXPERIMENTAL SECTION 2.1 Materials A 1.47 wt% aqueous suspension of CNF produced by TEMPO mediated oxidation of wool pulp was purchased from Tianjin Haojia Cellulose (China) Co. Ltd. 3-Hydroxytyramine Hydrochloride (dopamine hydrochloride) was purchased from Adamas Reagent Co., Ltd. All the other chemical agents and solvents used in present work were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received. 2.2 Preparation of cellulose-based sponges PdCl2 (20 mg, 0.11 mmol) and dopamine hydrochloride (15.5 mg, 0.08 mmol) were added into CNF suspension (10 g, 1.47 wt%) under magnetic stirring at ambient temperature. A suitable amount of tris(hydroxymethyl)metyl aminomethane was added to adjust the pH to 8.5 and the solution was stirred for 8 h. 0.147 g GPTMS was added dropwise into the suspension. Thereafter, the suspension was frozen with liquid nitrogen from bottom-up and the resulted frozen gel was subjected to freeze-drying on a Labconco FD5-3 freeze-dryer at -55 oC for 24 h. Consequently, the cellulose-based sponge was cured for 30 min at 110 oC in a vacuum oven to ensure sufficient cross-linking38 followed by rinsing with deionized water to remove unbound materials. Then the sample was further purified by extracting absorbed water with acetone successively using a Soxhlet extractor. Finally, the sample was dried for 1 h at 60 oC in a vacuum oven. The sample was coded as Pd NPs@CS. 2.3 Characterizations The apparent density of the sponge was determined according to the ISO standard 845:2006. The specific surface area of the samples was determined at 77 k by 4

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Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption measurement using a Micromeritics Tristar II 3020 system. The Pd content on the cellulose sponges and in solution was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman Prodigy, USA) according to general rules (JY/T 015-1996).The crystal structures and phase data for the Pd NPs were determined using wide angle X-ray diffraction (XRD, Rigaku Ltd., Japan) with Cu Kα (λ = 1.54056 Å). The morphology of the sponge was characterized by Hitachi TM-1000 scanning electron microscopy (Japan). The size and structure of palladium nanoparticles (Pd NPs) were obtained by high-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL, Japan). FT-IR was carried out on a PerkinElmer Spectrum-Two (American) equipped with an ATR accessory in the range of 4000-500 cm-1 at a resolution of 4 cm-1. The thermogravimetric analysis (TGA) was performed on a TG 209 F1 thermal analysis (NETZSCH, Germany) under nitrogen atmosphere from ambient temperature to 600 oC at a heating rate of 10 oC /min in an open alumina pan. The compressive strength of the sponge was evaluated on universal testing mashing (HY 940F, China) at a compression of 1 mm/min in ethanol-water (vol : vol = 1 : 1), the maximum compression strain (ε) was set to 80%. 2.4 Catalytic performances of the Pd NPs@CS Suzuki reactions: Taking the reaction of phenylboronic acid and bromobenzene as an example: a mixture of bromobenzene (0.7 mmol), phenylboronic acid (1.05 mmol), K2CO3 (1.05 mmol) and 15 mg Pd NPs@CS in 3 mL of ethanol/H2O (vol : vol = 1 : 1) was stirred at 65 oC for 3 h. GC/MS analysis was used to monitor the reaction progress. In the recycling experiments, Pd NPs@CS catalysts were taken out of the solution and rinsed with ethyl acetate and ethanol after each reaction cycle before being used in the next cycle. Heck reaction:

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Taking the reaction of Bromobenzene and styrene as an example. Bromoenzene (0.5 mmol), styrene (0.75 mmol), K2CO3 (0.75 mmol) and 10 mg Pd NPs@CS in 3 ml DMF was stirred at 140 oC for 6 h. GC/MS analysis was used to monitor the reaction progress.

3. RESULTS AND DISCUSSION The synthesis of Pd NPs@CS is carried out according to the scheme in Figure 1. PDA, the self-polymerization product of dopamine, is a versatile agent for surface modification of various materials. The catechol moiety of PDA readily form chelation with metal ions, allowing for easy in situ nucleate and growth of metal nanoparticles on PDA-modified surfaces. As a result, these metal NPs are firmly immobilized onto the CS surface. GPTMS was employed to improve the mechanical properties of sponges as a cross-linker. Because the silicon hydroxyl formed from hydrolysis of the methoxysilyl moiety can condense with the hydroxyls on cellulose,39 while the epoxy group can react with amino groups of PDA.

Figure 1. Schematic of the stepwise formation of Pd NPs@CS.

The modified cellulose sponges were subjected to FT-IR, XRD, EDX, SEM, TEM , ICP-AES analyses and compress strain-stress tests. The average density of the composite sponge was found to be 27.07 (±0.3) mg/cm3 and the BET special surface area was calculated to be 10.3 m2/g (Figure S1). The cross-linking efficiency of the CNF/GPTMS/PDA system was studied by FT-IR (Figure 2a). The peaks at 2926 cm-1 and 2873 cm-1 are attributed to C-H stretching of the unhydrolyzed silyl methoxyls on GPTMS. The absorption peaks at 1255 cm-1, 906 6

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cm-1, and 852 cm-1 are characteristic for epoxy functionalities. The peak at 1517 cm-1 is attributed to vibration of the indole ring on PDA skeleton.37 These results implied the successful cross-linking of GPTMS and CNF. The apparent diminishing of characteristic epoxy peaks, and the appearance of indole ring stretching peak in the IR spectrum of Pd NPs@CS (blue line), confirmed that PDA and CNF were chemically joined up with GPTMS.

Figure 2. (a). FTIR of pure cellulose sponge (black line), GPTMS modified cellulose sponge (red line) and Pd NPs@CS (blue line); (b) XRD patterns of cellulose sponges with (CS) and without Pd NPs (Pd NPs@CS); (c) SEM image of Pd NPs@CS with elemental mapping images of (d) C, (e) O, (f) Si, (g) N and (h) Pd, respectively, on a cellulose sheet.

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To confirm the presence of Pd on the cellulose sponge, wide angle XRD measurements were employed. The XRD patterns of the sponges with and without Pd NPs are shown in Figure 2b. The broad peak at 2θ = 22.7° present in both spectra resulted from crystalline cellulose.18, 40 The peaks at 2θ = 40.0°, 46.5° and 67.8° correspond to diffractions from the (111), (200) and (220) lattice planes of the face-centred cubic Pd.11, 41 To further explore the distribution of Pd NPs, the sample was subjected to energy-dispersive X-ray spectroscopy (EDX). Elemental mapping images shown in Figure 2c-h show no obvious difference between the distribution of Pd and other elements, indicating the Pd NPs were homogeneously dispersed onto the cellulose sheet. Furthermore, the amount of Pd embedded was quantified by ICP-AES analysis to be 8.91 mg/g. With the successful immobilization of Pd NPs, their sizes and distribution as well as the morphology of the cellulose sponges were investigated. The morphology of the composite sponge was characterized by SEM and the results are shown in Figure 3(a-d). Highly porous structure with a narrow pore size distribution was observed for both the surface and the cross-section of the Pd NPs@CS. During the process of freezing, ice crystals grew and eventually produced a structure of anisotropic ice crystals surrounded by sheeted nanocellulose.42 The porosity, measured using a gravimetric method,43 of the present sponge is 96.2% (±0.6%). The average pore size on the surface of the sponge was around 20 µm on the surface 30 µm at the inner cross-sections (Figure 3c and 3d). The results of nitrogen adsorption and desorption showed that the adsorption isotherm fits with type Ⅳ with hysteresis loop at P/P0 > 0.2 (Figure S1), indicating the sponges was mesoporous with pore size ranging from 2 to 50 nm.44-45 The presence of nano-scale pores were also evidenced by the Barrett-Joyner-Halenda (BJH) pore size distribution results.

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Figure 3. SEM of the surface (a, c) and cross-section (b, d) of Pd NPs@CS, TEM images (e) and size distribution (f) of Pd NPs. The insets in (c) and (d) showing the respective SEM images with higher magnifications.

TEM images of Pd NPs@CS and the size distribution of Pd NPs are present in Figure 3e and 3f. The TEM image in Figure 3e clearly shows that the spherical Pd NPs were homogenously dispersed on the surface of the cellulose sheet. And these spherical Pd NPs had a narrow size distribution averaged at about 2.5 nm (average of 9

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200 NPs, Figure 3f). Very few Pd NPs were found larger than 5 nm (Figure S2a). The HRTEM image of Pd NPs (Figure S2b) shows well resolved lattice planes of Pd NPs with a spacing of 0.226 nm corresponding to the (111) diffraction plane. 11, 41, 46-47 The TEM, SEM and XRD results collectively confirm that the Pd NPs in situ formed with a narrow size distribution were successfully anchored on the surface of the thin cellulose sheet. Excellent mechanical properties were necessary for the sponge to ensure that the sponge can be used in a circulatory fashion. The compression stress-strain performance of Pd NPs@CS was evaluated in an ethanol-water mixture (vol : vol = 1 : 1), which is common media for cross-coupling reactions. As shown in Figure 4a, the shapes of these stress-strain curves are typical for open-cell foams:28, 48-51 and elastic behavior was observed at the compressive strain of less than 10%. When the compressive strain ranged from 10% to 60% cell collapse led to the reduction of stress and plastic stiffening. At high strains of above 60% sharp rise of stress was observed. The composite sponge could bear an 80% reduction in volume under applied pressure and recover to almost its original volume when the pressure was removed (Figure 4b). This result demonstrated the excellent flexibility and shape resilience of the sponge. Notably, the sponge is “machinable” in that it can be easily cut into nubbles without any structural deformation or collapse (Figure 4c). The excellent mechanical and shape resilience properties of the sponge both owe to the chemical crosslinks formed among GPTMS, CNF and PDA.

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Figure 4. (a) Compressive strain-stress curve of Pd NPs@CS; (b) Sequential photos of the sponge during the compression process and (c) Digital photograph of the prepared Pd NPs@CS.

Furthermore,

the

thermal

stability

of

the

catalyst

was

examined

by

thermogravimetric analysis. The result showed that Pd NPs@CS were stable up to 250 oC except for the mass loss around 100 oC due to evaporation of the absorbed water (Figure S3). The catalytic performance of the Pd NPs@CS was tested for Suzuki coupling reactions. Reaction of bromobenzene (0.5 mmol) and phenylboronic acids (0.6 mmol) in the presence of 1% mmol Pd NPs@CS was first examined. The plot of yield against reaction time is shown in Figure 5a. Maximum yield was achieved in 180 min. It was also demonstrated that the reaction could be stalled and restarted by removing and restoring the catalyst, confirming the critical role of the Pd NPs@CS catalyst. These results also suggest the Pd NPs@CS catalyzed reactions are “controllable”.

Figure 5. Catalytic activity (a) and recycling (b) of Pd NPs@CS in the cross-coupling reaction of aryl halide and aryl boronic acid.

With the catalytic activity confirmed the generality of Pd NPs@CS as a Suzuki catalyst was tested against a selection of arylhalides and arylboronic acids. The results are summarized in Table 1. Excellent yields were obtained for all the reactants examined. 11

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Table 1. Suzuki cross-coupling reactions of various aryl halides and arylboronic acids using Pd NPs@CS as catalyst. a OH X +R

Pd NPs@CS, 1 mmol% R

B OH

a

. Reaction conditions: aryl halide (0.5 mmol), aryl boronic acid (0.6 mmol), ), K2CO3(0.75 mmol), Pd NPs@CS (1 mmol% ),

and 3.0 ml of water-ethanol (1:1 (v/v)) at 65 oC. b

. Yield determined by GC-MS.

To further explore the generality of this catalyst, another cross-coupling reaction, Heck reaction, was tried. As shown in Table 2, both phenyl iodide and phenyl bromide form the corresponding Heck coupling products with a selection of styrene derivatives in reasonable yields (62%-79%). Table 2. Heck cross-coupling reactions of various aryl halides and aryl ethylene using Pd NPs@CS as catalyst. a

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R X

Pd NPs@CS 1 mmol%

+

R

a

. Reaction conditions: aryl halide (0.5 mmol), aryl ethylene (0.6 mmol), ), K2CO3(0.75 mmol), Pd NPs@CS (1 mmol% ), and

3.0 ml DMF at 140 oC. b

. Yield determined by GC-MS.

The most prominent advantage of heterogeneous catalysts is easy separation and reusability. The porous Pd NPs@CS produced herein could be easily drawn out from the reaction mixture with tweezers, rinsed with ethyl acetate and ethanol, and reused in the next cycle. The reaction of bromobenzene and phenylboronic acid was chosen to test the recyclability of catalyst and the results are plotted in Figure 5b. It was demonstrated that after up to seven reaction cycles, both the activity (Figure 5b) and the chemical contents (Figure S4) of the catalyst remained unchanged. Since Pd NPs are responsible for the catalytic reactivity, the leaching of Pd from the cellulose carrier, since it was a major concern in heterogeneous catalysis, was also studied. The reaction mixtures were collected and subjected to ICP-AES analysis. Results showed that in all cases, the Pd content was below detectable limit. To macroscopic insights the leaching of Pd was negligible, we took out the catalyst of the reaction of bromobenzene and phenylboronic acid and explored the change of the yield of the product. As it shown in Figure 5a (red line), no change in conversion was observed when the catalyst was separated. When the catalyst was added into 13

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before-mentioned solution again, the reactants underwent smooth conversions again. These results suggest that Pd NPs was firmly immobilized on the cellulose sponge and the catalyst could let reactions “controllable”. Cellulose supported Pd NPs can also be used to catalyze a variety of reactions such as reduction of methylene blue and 4-nitrophenol,

52

Sonogashira reactions,2

palladium-catalyzed C−H arylation reactions53-54 etc. The sponge prepared herein can also be used as support for other metal nanoparticle catalysts23 such as Nano Au, Ag and Pt using a similarly procedure. The approach presented in this paper is well suited for the synthesis of a wide range of recyclable metal nanocatalysts.

4. CONCLUSIONS Sustainable Pd catalyst supported on highly porous and flexible cellulose sponge was prepared. It was demonstrated that the porous materials had a narrow pore size distribution. XRD and TEM results revealed that the in situ formed spherical palladium nanoparticles (Pd NPs) had a narrow size distribution and were homogenously dispersed on the surface of cellulose sheet. The Pd NPs@CS shown remarkable catalytic activity and excellent recyclability in Suzuki and Heck cross-coupling reactions, during which minimum/next to none Pd leaching was observed. This approach offers a general way of developing recyclable, non-leaching heterogeneous transition metal catalyst systems containing metal nanoparticles.

 ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Nitrogen adsorption and desorption isotherms and Barrett-Joyner-Halenda (BJH) pore size distribution of Pd NPs@CS (Figure S1); TEM (a) and HRTEM (b) images

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of Pd NPs (Figure S2); TGA plots of Pd NPs@CS (Figure S3) and FTIR of Pd NPs@CS before and it after seven cycles of catalytic tests (Figure S4).

 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +86 21 67792720. Fax: +86 21 67792707. *E-mail: [email protected]. Tel.: +86 21 67792605. Fax: +86 21 67792707. Author Contributions The manuscript was written with contributions from all of the authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

 ACKNOWLEDGEMENTS This work was financially supported by National Natural Science Foundation of China (No.51403035), and the Fundamental Research Funds for the Central Universities (No.16D110510).

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